Abstract: B cells are responsible for maintaining humoral immunity. However, a comprehensive spatial map of murine and human B cell subsets across tissue compartments in the body is lacking. A barrier to these studies has been the lack of methods for comprehensive analysis of large numbers of cells from many different tissues and also access to rare human specimens. Here we use advanced computing and unbiased approaches to organize B cell subset and their signaling properties in mouse and man. We will test the hypothesis that B cell subset composition can be predicted by tissue site and that long-lived PCs in different tissues sites have distinct regulatory programs that reflect their unique microenvironments. In Aim 1, we will establish the population structure of murine and human B lineage subsets throughout the body using unbiased approaches. In Aim 2, we will establish the tissue-specific regulatory programs and clonal relationships of murine long-lived PC (LLPC) derived from two distinct microenvironments, BM versus gut. Results from this study will reveal the global population structure of B subsets responsible for humoral immunity and provide insights into how B cells control infections regionally and systemically.